1. Field of the Invention
The present invention relates generally to devices and methods for filtering of optical signals, and more specifically to optical filtering using optical fiber tapers and bitapers.
2. Description of the Related Art
Fiber tapers and bitapers have found a wide range of important applications in optical communication and sensing, including as fused couplers, narrow-band filters, for mode-matching between waveguides, and for evanescent mode coupling. See, e.g., A. W. Snyder, “Coupling of modes on a tapered dielectric cylinder,” IEEE Trans. Microwave Theory and Techniques, 1970, Vol. 18, pp. 383-392; D. T. Cassidy, D. C. Johnson, and K. O. Hill, “Wavelength-dependent transmission of monomode optical fiber tapers,” Appl. Opt., 1985, Vol. 24, pp. 945-950; S. Lacroix, F. Gonthier, and J. Bures, “All-fiber wavelength filter from successive biconical tapers,” Opt. Lett., 1986, Vol. 11, pp. 671-673; D. Marcuse, “Mode conversion in optical fibers with monotonically increasing core radius,” J. Lightwave Technol., 1987, Vol. B, pp. 125-133; J. D. Love et al., “Tapered single-mode fibres and devices, Part I,” IEEE Proc., 1991, Vol. 138, pp. 343-354; R. J. Black et al., “Tapered single-mode fibres and devices, Part II,” IEEE Proc., 1991, Vol. 138, pp. 355-364.
In the portion of a conventional fiber bitaper that necks down, the geometrical perturbation of the core couples the fundamental core mode (HE11) to one or more cladding modes (e.g., HE12, HE13, etc.). Because the coupled modes have different phase velocities, as they travel along the central portion of the bitaper, they accumulate a differential phase shift Δφ. Past this region, the bitaper diameter increases and the two (or more) modes exchange energy again. Depending on Δφ, power is either coupled back to the core mode (taper then has a high transmission) or remains in the cladding modes (low transmission). Because this coupling is wavelength dependent, the transmission spectrum exhibits a nearly periodic modulation. For conventional fiber tapers, the period is typically 10-40 nanometers and the modulation depth ranges from 0 to about 20 dB, depending on the strength of the taper (which can be defined as the ratio between the diameter of the neck and the original fiber diameter).